The present invention relates to fuel cells assembled together to form a fuel cell power plant suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to an operating system for a fuel cell power plant that minimizes free water and that also utilizes an antifreeze solution passing through the plant to remove heat.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cM.E.A.xe2x80x9d, and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid. In fuel cells containing porous reactant flow fields, as described in U.S. Pat. No. 4,769,297, owned by the assignee of all rights in the present invention, a portion of the water maybe alternatively removed as a liquid through the porous reactant flow field to a circulating cooling fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEN. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells assembled together in a well known fuel cell stack with additional components to form a fuel cell power plant in order to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions. Known fuel cell power plants typically utilize a coolant or thermal management system supplying a flow of cooling fluid through the fuel cell and other plant components to maintain the cell within an optimal temperature range and efficiently distribute heat. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize an antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in such coolant systems.
However, such antifreeze solutions are known to be adsorbed by and poison the catalysts that form electrodes. Furthermore, those antifreeze solutions have low surface tensions which results in the solutions wetting any wetproofed support layers adjacent cell catalysts, thereby impeding diffusion of reactant fluids to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of those antifreezes is too high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams or from steam produced in boilers of fuel processing components of fuel cell power plants. Therefore coolant systems of fuel cells that utilize an antifreeze solution are known to be sealed from the electrodes, so that the solution is not in direct fluid communication with the electrode catalysts.
Sealing the coolant system from direct fluid communication with the cell and hence with the product water formed at the cathode electrode results in decreased cell performance due to less efficient removal of the product water. Fuel cell power plants that utilize sealed coolant plates typically remove product water as an entrained liquid. This requires a tortuous serpentine flow path with a resultant high pressure drop. An example of such a cell is shown in U.S. Pat. No. 5,773,160. That type of cell is not suitable for operating at near ambient reactant pressures which is a preferred operating pressure for many fuel cell systems. The decreased performance of cells with sealed coolant plates requires that additional cells be used to satisfy the design power requirement. The additional cells combined with heavier, sealed coolers results in an increase in weight and volume of a fuel cell power plant which is undesirable for powering a vehicle.
Additionally, where a fuel cell power plant powers a vehicle, the atmosphere serving as a process oxidant stream directed into contact with the cathode electrode will vary significantly in humidity. Consequently, it is known to undertake substantial efforts to humidify the process oxidant and reducing fluid reactant streams in order to minimize water loss from the PEM electrolyte. Known efforts include recycling some of the product water from the cell, and/or directing some of the cooling fluid within the coolant system as a vapor into the process oxidant and/or reducing fluid streams entering the fuel cell. However, with known fuel cells, the humidity enhancing fluid must be free of any antifreeze solutions in order to prevent the antifreeze from poisoning the catalysts. Known fuel cells therefore utilize sealed coolant systems that are isolated from humidification systems. For example, one known fuel cell humidification systems utilizes complex, heavy and large membrane barrier components consisting of uncatalyzed PEM cells upstream of catalyzed cells in order to isolate any antifreeze solution within the cooling fluid or within the product water mixed with cooling fluid from contact with the electrode catalysts. Such efforts to isolate the antifreeze solution add to the cost, weight and volume of the fuel cell.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize fuel cell power plant weight and space requirements, the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant.
An additional difficulty in maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to process hydrocarbon fuels, such as methane, natural gas, methanol, gasoline, diesel fuel, etc., into an appropriate reducing fluid that provides a hydrogen rich fluid to the anode electrode. Such fuel processing components of a fuel cell power plant typically include a boiler that generates steam; a steam duct into which the hydrocarbon fuel is injected; and an autothermal reformer that receives the steam and fuel mixture along with a small amount of a process oxidant such as air and transforms the mixture into a hydrogen rich reducing fluid appropriate for delivery to the anode electrode of the fuel cell. The fuel processing components also include system water and energy requirements that are part of an overall water balance and energy requirement of the fuel cell power plant. Water made into steam in the boiler must be replaced by water recovered from the plant such as by condensing heat exchangers in the cathode exhaust stream and associated piping.
A further problem associated with using fuel cell power plants in transportation vehicles arises from a need to have such vehicles capable of commencing immediate operation without any significant warm-up period. Use of a boiler to generate steam in the fuel processing system, however, requires either a warm up period; a substantial energy cost to rapidly initiate boiling; or, an alternative fuel source for the cells or power source for the plant until the fuel processing components are capable of generating adequate fuel.
In some prior art fuel cell power plants utilizing fuel reformers, it is known to direct all or a portion of a cathode exhaust stream into the fuel reformer to thereby minimize water recovery apparatus like condensing heat exchangers, such in as U.S. Pat. No. 3,976,507 issued on Aug. 24, 1976 to Bloomfield, and U.S. Pat. No. 4,128,700 issued on Dec. 5, 1978 to Sederquist. Both of those patents are assigned to the assignee of the present invention, and are hereby incorporated herein by reference. While such older patents show minimization of water recovery efforts, those and other known fuel cell power plants that direct cathode exhaust streams directly into fuel processing components are typically complex pressurized plants that involve use of supercharging turbines mechanically linked to compressors. Therefore those plants have not been shown to be appropriate for utilization in transportation vehicles which require a fuel cell power plant that must operate within water balance through a wide variety of environmental conditions.
A further difficulty associated with cooling fuel cells wherein the cooling fluid contacts cell components is limiting conductivity of the cooling fluid to avoid shunt current corrosion that results from a conductive cooling fluid providing a conductive bridge between cell components. Where the cooling fluid includes water, or similar solvents dissolved metals and other ions must be removed, such as by passing the cooling fluid through, a demineralizer as shown in U.S. Pat. No. 4,344,850 to Grasso, which Patent is owned by the assignee of all rights in the present invention, and which Patent is hereby incorporated herein by reference. However, when such a demineralizer or similar water treatment system is utilized within a fuel cell power plant in a vehicle, the cooling fluid within the water treatment system components must also be protected against freezing. If the cooling fluid were to freeze, it could cause mechanical damage to components of a water treatment system such as a demineralizer; it would require melting during a start-up procedure; and, where a demineralizer includes ion exchange resin beads, the freezing and thawing cooling fluid could cause break up of the beads, resulting in a high pressure drop and impeded flow within the demineralizer. Consequently, the water treatment system for treating the cooling fluid must be freeze protected without adding significant further weight and cost to the fuel cell power plant.
Accordingly there is a need for a fuel cell power plant having fuel processing components that may be operated in sub-freezing conditions that does not require isolating an antifreeze cooling fluid from the cathode and anode catalysts within a sealed coolant system; that minimizes free water within the system that may be frozen whenever the fuel cell is shut down and not operating while subjected to sub-freezing temperatures; that maintains a self-sufficient water balance during operation; that can achieve a rapid generation of power without a requirement of first melting substantial amounts of ice or frozen water; and, that does not require significant increases in weight, volume or cost of the plant.
An operating system for a direct antifreeze cooled fuel cell power plant is disclosed for producing electrical energy from reducing and process oxidant fluid reactant streams. The system includes at least one fuel cell for producing electrical energy from the reducing and process oxidant fluid reactant streams; a thermal management system that directs flow of a cooling fluid for controlling temperature within the plant including a porous water transport plate adjacent and in fluid communication with a cathode catalyst of the fuel cell and including a direct antifreeze solution passing through the water transport plate; fuel processing components including an autothermal reformer and a burner for processing a hydrocarbon into the reducing fluid; and, a split oxidant passage that directs the process oxidant stream through the fuel cell and that also directs a reformer feed portion of the process oxidant stream through at least one reformer feed portion of the split oxidant passage into fluid communication with the fuel processing components, and that also directs an exhaust portion of the process oxidant stream through an exhaust branch of the split oxidant passage into a plant exhaust passage. Reformer feed branches of the split oxidant passage may direct the process oxidant stream into fluid communication with the fuel processing components before or after the process oxidant stream passes through the fuel cell.
In a first alternative embodiment of the operating system, an anode exhaust passage is included that receives an anode exhaust stream exiting the fuel cell and directs the anode exhaust stream into the burner, then directs the combusted burner exhaust stream from the burner into heat exchange relationship with the reformer feed portion of the process oxidant stream within the reformer feed branch of the split oxidant passage, and then directs the combusted burner exhaust stream into the plant exhaust passage to mix with the exhaust portion of the process oxidant stream to become a plant exhaust stream. In an additional alternative embodiment, the system includes a direct mass and heat transfer device secured in fluid communication with both an oxidant inlet that directs the process oxidant stream into the split oxidant passage upstream of the fuel cell and also with the plant exhaust passage so that the device directly transfers through a mass transfer medium mass and heat such as water vapor exiting the plant in a plant exhaust stream within the plant exhaust passage back into the plant within the process oxidant stream, wherein the mass transfer medium may be the direct antifreeze solution cooling fluid directed from the thermal management system to pass through the direct mass and heat transfer device. In yet a further alternative embodiment, the system may include a fuel processing thermal exchange loop for removing heat from the fuel processing components that may also be secured in heat exchange relationship with the thermal management system.
In an additional alternative embodiment the operating system for a direct antifreeze cooled fuel cell power plant includes a water treatment system having a demineralizer in fluid communication with the cooling fluid, and having a degasifier also in fluid communication with the cooling fluid that passes the cooling fluid in mass transfer relationship with the process oxidant stream so that dissolved gases in the cooling fluid transfer from the cooling fluid into the process oxidant stream.
In a further embodiment, the direct antifreeze solution passing through the water transport plate may be directed to flow at a pressure that is less than a pressure of the process reactant streams passing adjacent the water transport plate. A preferred fuel cell operates at near ambient pressure and the process oxidant stream and reducing fluid stream are pressurized to 1 to 2 pounds per square inch gauge (hereafter xe2x80x9cPSIGxe2x80x9d) above ambient pressure, while the direct antifreeze solution is directed to flow through the water transport plate at about 1 to 2 PSIG below ambient pressure. Such a positive pressure differential between the process oxidant stream and the antifreeze solution within the water transport plate further assists movement of product water formed at a cathode catalyst of the fuel cell into the water transport plate. The positive pressure differential also limits movement of any liquid antifreeze solution flowing within the water transport plate from flowing out of the water transport plate into the higher pressure process reactant streams passing within reactant flow fields defined adjacent to and/or within fluid communication with the water transport plate.
The direct antifreeze solution of the invention may be any organic antifreeze solution that is non-volatile and does not wet a hydrophobic substance such as xe2x80x9cTeflonxe2x80x9d at cell operating temperatures. For purposes herein, xe2x80x9cnon-volatilexe2x80x9d is defined to mean that the antifreeze solution sustains a loss of less than 10% of its antifreeze for every 500 operating hours of the fuel cell power plant at fuel cell operating temperatures. Alternatively, a first preferred direct antifreeze solution may be a special direct antifreeze solution having the following characteristics: 1. a freezing point of at least xe2x88x9220 degrees Fahrenheit (hereafter xe2x80x9cxc2x0F.xe2x80x9d); 2. a surface tension greater than 60 dynes per centimeter (hereafter xe2x80x9cdyne/cmxe2x80x9d) at a cell operating temperature of about 150xc2x0 F.; 3. a partial pressure of antifreeze above the solution at about 150xc2x0 F. that is less than 0.005 mm of mercury (hereafter xe2x80x9cmm Hgxe2x80x9d); and, 4. that is capable of being oxidized by catalysts of the fuel cell at fuel cell voltages. A second preferred antifreeze solution may be an alkanetriol direct antifreeze solution, and in particular an alkanetriol selected from the group consisting of glycerol, butanetriol, and pentanetriol. The direct, special and alkanetriol direct antifreeze solutions minimize movement of the antifreeze as a vapor out of the water transport plate into contact with the cathode or anode catalysts, and also minimize direct antifreeze solution loss from the thermal management system, direct mass and heat transfer system, and water treatment system of the power plant as well as from any other fuel cell components such as the plant exhaust stream exiting the cell.
Accordingly it is a general object of the present invention to provide an operating system for a direct antifreeze cooled fuel cell power plant that overcomes deficiencies of the prior art.
It is a more specific object to provide an operating system for a direct antifreeze cooled fuel cell power plant that eliminates need for a separate sealed thermal management system for operation in sub-freezing conditions.
It is another object to provide an operating system for a direct antifreeze cooled fuel cell power plant that processes a hydrogen rich reducing fluid without any free liquid water and without a boiler for generating steam.
It is a further object to provide an operating system for a direct antifreeze cooled fuel cell power plant that transfers water exiting a fuel cell of the plant in oxidant and anode exhaust streams to components for processing hydrocarbon fuels utilized by the fuel cell.
It is yet another object to provide an operating system for a direct antifreeze cooled fuel cell power plant that eliminates any need for uncatalyzed membrane barrier components of a humidification system between an antifreeze cooling fluid and the fuel cell.
It is another object to provide an operating system for a direct antifreeze cooled fuel cell power plant that minimizes free liquid water that may freeze when the fuel cell is not operating in sub-freezing conditions.
It is yet a further object to provide an operating system for a direct antifreeze cooled fuel cell power plant that directly transfers mass and heat leaving the power plant back into the plant through a mass transfer medium supplied from a thermal management system.
These and other objects and advantages of the present operating system for a direct antifreeze cooled fuel cell power plant will become more readily apparent when the following description is read in conjunction with the accompanying drawings.